As I understand it the "Intelligence Corps" of WW1 consisted entirely of men serving in their own regimental uniforms, badges and buttons. To quote a very informative post on the Great War Forum:

The permanent Intelligence Corps was not ‘formed’ and badged until July 1940 and the reason for that late recognition was probably prejudice. There had previously been ad hoc Intelligence units (and ad hoc Intelligence Corps in WWI) but the concept of a permanent Intelligence setup had been considered undesirable – any hint that you may call on covert activities to fight a war was anathema to many at the top.